The present invention relates to a Schottky gate type field-effect transistor (FET) with the Schottky junction as the gate structure, and particularly, to a diamond Schottky gate type FET having a structure comprising diamond films formed by chemical vapor deposition (CVD).
2. Description of the Prior Art
Diamond has a high thermal conductivity, an excellent stability against heat, and a large band gap. It is electrically insulating but becomes semiconducting upon doping. Therefore, diamond is expected to be used for semiconducting devices which can be operated in a high electric power/high temperature regime. By the advent of the synthesis of diamond films by chemical vapor deposition (CVD), it has now become possible to grow B (boron)-doped p-type semiconducting diamond films and Si (Silicon)-doped n-type semiconducting diamond films.
Research and development effects have been made to develop semiconducting devices employing such semiconducting diamond films. The present applicant has proposed an MIS type diamond FET having a metal electrode/insulating diamond layer/semiconducting diamond layer structure (Japanese Patent Laid-open No. hei 2-63827). FIG. 5 is a schematic cross-sectional view of the conventional MIS type diamond FET (hereafter referred as "MISFET).
In FIG. 5, an insulating diamond under layer 52 is formed on a silicon substrate 51. Three layers are formed on the insulating diamond under layer 52 in the order of; a B-doped p-type semiconducting diamond layer 53 as an active layer, a Si-doped n-type semiconducting diamond layer 54a as an active layer to be a source area, and a Si-doped n-type semiconducting diamond layer 54b as an active layer to be a drain area. Furthermore, an insulating diamond layer 55 is formed on the p-type semiconducting diamond layer 53.
A source electrode 56 having a two-layer structure of Ti (Titanium) and Au (Gold) is provided on the surface of the n-type semiconducting diamond layer 54a, to form an ohmic contact therebetween. Also, a drain electrode 57 made of Au/Ti is provided on the surface of the n-type semiconducting diamond layer 54b, to form an ohmic contact therebetween. Furthermore, a gate electrode 58 made of Au/Ti is provided on the surface of the insulating diamond layer 55.
The MISFET described above can normally operate in high temperatures.
However, the MISFET is disadvantageous in that the thermal expansion coefficient of the metal electrodes are quite different from that of diamond. When it undergoes a heat cycle from about 500.degree. C. to room temperature (about 25.degree. C.), there occur dislocation and defects due to the thermal strain in the semiconducting diamond layers and the insulating diamond layers. Consequently, it cannot normally operate and further introduces a fear of the electrodes peeling off.